Gas-turbine engines (also generally termed “turbines”) can be relatively clean, efficient, and less costly to construct than other power-generation alternatives and offer a blend of operational attributes that set them apart from the more traditional power-generation plants. Turbines generally include a compressor for pressurizing air and a combustor for mixing the pressurized air with fuel. Multiple flames within the combustor ignite the fuel-air mixture to generate a heated-gas exhaust. The heated-gas exhaust is passed into a turbine to generate power.
During turbine operation to generate power, the combustor flame burns constantly. An unintended termination of the combustor flame can occur, however, and is referred to as a “flameout.” Flameout can occur, for example, if the fuel-to-air ratio is or becomes too rich or too lean to sustain combustion in the combustor.
To control fuel-to-air ratios at desired levels to prevent flameouts and other undesired consequences, turbine engines typically include controllers that commonly have employed a control strategy in which the fuel supply and the air supply to the turbine are separately controlled by reference to different measured turbine-performance parameters. For example, in a typical gas-turbine controller, fuel supply to the turbine is controlled primarily by a feedback loop that seeks to match the power output from the turbine with load demand on a power generator (e.g., electric-power generator) that is being driven by the turbine. This feedback is typically accomplished by monitoring the rotational speed of the turbine and by increasing or decreasing the fuel supply to the turbine to increase or decrease, respectively, the rotational speed as needed.
Other control systems have utilized the exhaust from the turbine for estimating fuel-to-air ratios in the combustor. These control systems typically examine the difference between turbine-exhaust temperature as measured and a reference-temperature value. A change in exhaust temperature causes the controller to change airflow to, and thus the fuel-to-air ratio within, the turbine combustor.
Because the types of adjustments summarized above are based upon post-combustion temperature of the exhaust from the turbine, these types of control mechanisms introduce a lag between exhaust-temperature assessment and correction of the fuel-to-air ratio based upon the assessment. This lag can present a particularly significant challenge in the event of a reduction in load demand. For example, if the magnitude of the change in load is sufficiently great, the control-system lag may cause one or more combustors in the turbine to experience flameout if the fuel-to-air mixture becomes too lean or too rich. If a sufficient number of combustors in the turbine experience flameout, the turbine may shut down and cease supplying power. Restarting the turbine often takes a substantial amount of time and effort, and the flameout/restart process can impose undesired thermal and mechanical wear on the turbine.
Similar concerns have also lead to complications in controlling fuel-to-air ratios in other combustor environments. Examples include boilers, furnaces, and other engines such as conventional internal-combustion engines, which are widely used in automobiles, trucks, motorcycles, and boats.
In conventional internal-combustion engines, for example, control devices are often used for assessing the oxygen content in the exhaust gas. For this purpose, oxygen-measurement probes have been used to provide a voltage signal corresponding to the partial pressure of oxygen in the exhaust gas. The voltage signal increases whenever the partial pressure of oxygen changes from excess oxygen to deficient oxygen in the exhaust, or vice versa. The output signals produced by the oxygen-measurement probes are evaluated by a controller that responds to changes in the partial pressure of oxygen by adjusting the fuel-to-air mixture. Thus, the controller assesses exhaust outside of the engine combustor as the controller seeks to determine or control activity occurring within the engine combustor. This post-combustion type of control system can introduce errors in making the assessment of oxygen content and can introduce time delays in effecting desired adjustments within the combustor based on the assessed oxygen content.
The fuel-to-air ratio usually is not the only aspect of concern in maintaining combustion in engines, furnaces, boilers, etc. Other concerns include providing efficient and reliable power generation while simultaneously seeking to minimize undesirable engine wear and noxious emissions from the combustion process occurring in the engine. Exemplary noxious emissions in the exhaust from a gas turbine include nitrogen oxides (NOx), unburned hydrocarbons, carbon monoxide (CO), and other emissions. Controlling these undesirable emissions requires control of the fuel-to-air ratio of the combustible mixture being fed into the combustion chamber of the turbine.
One conventional approach to reducing noxious emissions from the turbine has been to configure the turbine such that, whenever the turbine is operating under a full-load condition, the fuel-to-air ratio entering the turbine has a particular equivalence ratio (i.e., the actual fuel-to-air ratio divided by a stoichiometric ratio of fuel to air that is based on theoretically complete combustion) that corresponds to a desired fuel-to-air point situated between the lean-flameout point (at which flameout occurs because the fuel-to-air ratio is too lean) and the rich-flameout point (at which flameout occurs because the fuel-to-air ratio is too rich). For reasons of reducing emissions and improving fuel economy, turbines are commonly operated with a fuel-to-air ratio of less than unity, i.e., a fuel-and-air mixture that is leaner than the stoichiometric fuel-to-air ratio. However, whenever the fuel-and-air mixture is too lean, carbon monoxide is produced, and whenever the fuel-and-air mixture is too rich, the exhaust includes unburned hydrocarbons, which is wasteful of fuel. Thus, accurate maintenance of predetermined fuel-to-air ratios within prescribed limits is important not only for controlling emissions of pollutants from the engine but also for operating the engine reliably without causing undue damage as noxious exhausts are minimized.
Background references include Kleppe, Engineering Applications of Acoustics, Artech Press, Boston, London, 1989, Kleppe et al., “The Application of Acoustic Pyrometry To Gas Turbines and Jet Engines,” Proceedings 34th AIAA/ASME/SAE/ASEE Joint Propulsion Conference, Cleveland, 98-3611, pp 1-10, July, 1998; Kleppe, “Acoustic Pyrometry: A Historical Prospective,” Proceedings 44th International Instrumentation Symposium, Reno, pp 504-512, May, 1998; Kleppe et al., “High Temperature Gas Measurements in Combustors Using Acoustic Pyrometry Methods,” Proceedings 47th International Instrumentation Symposium, Denver, pp. 6-10, May 6-10, 2001; Verhage et al., “Damage of Hot Gas Components in Gas Turbines Due to Combustion Instabilities,” Proceedings ECOS2000, Vol. 4, Eurotherm 66 and 67 Seminars, Universiteit Twente, 2000; and Schmidt, “Multiple Emitter Location and Signal Parameter Estimation,” IEEE Trans. Antennas Propogat., Vol. AP-34, pp. 276-280, 1986.